1. Field of the Invention
The present invention relates to a cascade scanning optical system having a pair of laser scanning optical systems which are arranged along the main scanning direction and controlled to operate in synchronization with each other so as to realize a wide scanning line. More specifically the present invention relates to a scanning apparatus having such a cascade scanning optical system in which the rotation of a polygon mirror of one laser scanning optical system is synchronized with the rotation of a polygon mirror of the other laser scanning optical system to prevent a pair of scanning lines that are to be aligned, respectively generated by the pair of laser scanning optical systems, from being deviated from each other in the main scanning direction.
2. Description of the Related Art
A cascade scanning optical system having a plurality of laser scanning optical systems arranged along the main scanning direction to realize a wide scanning line is known. Such a type of scanning optical system is disclosed in Japanese Laid-Open Patent Publication No.61-11720, published on Jan. 20, 1986. This publication discloses a cascade scanning optical system having a pair of laser scanning optical systems each having a laser beam emitter, a polygon mirror serving as a deflecting device, an f.theta. lens, etc. The pair of laser scanning optical systems are synchronously driven to emit respective scanning laser beams to a photoconductive surface (scanning surface) of a photoconductive drum on a common line thereon extending in parallel to the axial direction of the photoconductive drum. The pair of scanning laser beams respectively scan two adjacent ranges of the common line on the photoconductive surface so as to scan the photoconductive surface of the photoconductive drum in the main scanning direction in a wide range.
There is a fundamental problem to be overcome in such a type of cascade scanning optical system. Namely, how can a scanning line, made on the photoconductive drum by the scanning laser beam emitted from one laser scanning optical system of the cascade scanning optical system, be accurately aligned with another scanning line, made on the photoconductive drum by the scanning laser beam emitted from another laser scanning optical system of the cascade scanning optical system, so that the scanning lines are not apart from each other nor overlap each other in the main scanning direction, i.e., so as to form a straight and continuous scanning line using a combination of separate scanning lines.
The f.theta. lens or lens group, provided in each of the pair of laser scanning optical systems, is often made of plastic for the purpose of reducing the cost of production. It is well-known that a plastic lens is liable to be deformed and also the refractive power thereof is liable to change, due to a variation in temperature, humidity, etc. Therefore, in the case where such a variation in temperature, humidity, etc. occurs, the length of the scanning line made by a rotation of the corresponding polygon mirror by a specified angle of rotation (i.e. by a predetermined time interval) may vary even if the polygon mirror rotates at a constant speed. When such a variation in length of the scanning line occurs, the pair of scanning lines, which are to be aligned to form a straight and continuous scanning line, are apart from each other or overlap each other in the main scanning direction. In FIG. 2, respective scanning laser beams deflected by polygon mirrors 24A and 24B to scan a drum 10 (the photoconductive surface of the drum 10) are represented by solid lines 1, one-dot-chain lines 2 and broken lines 3. The scanning laser beams drawn by the solid lines 1 are of those when no deformation or variation in the refractive power occurs in each f.theta. lens and accordingly form a straight and continuous scanning line without the pair of scanning lines being apart from each other or overlapping each other in the main scanning direction (the horizontal direction as viewed in FIG. 2). The scanning laser beams drawn by the one-dot-chain lines 2 are of those when the length of each of the pair of scanning lines decreases due to a deformation in each f.theta. lens and accordingly form a discontinuous scanning line with the pair of scanning lines being apart from each other in the main scanning direction. The scanning laser beams drawn by the dotted lines 3 are of those when the length of each of the pair of scanning lines increases due to a deformation in each f.theta. lens and accordingly form a straight and continuous scanning line but the pair of scanning lines overlap each other in the main scanning direction.